In many industries strong wastewaters are produced which contain high concentrations of certain undesirable solutes the concentrations of which have to be substantially reduced before they are discharged to the environment. Said undesirable solutes include organic compounds, ammoniacal nitrogen, sulfides, and cyanides. The aforementioned strong wastewaters typically have high concentrations of dissolved solids such that they are difficult to treat using conventional wastewater treatment techniques. Frequently the undesirable solutes that need to be treated in such strong wastewaters are poisonous to the bacteria essential to biological treatment processes so even if they are diluted with other wastewaters, in order to reduce their strength, they will still disrupt and decrease the effectiveness of biological waste water treatment processes. As environmental regulations become more restrictive worldwide it is becoming very difficult to meet the required discharge limits if the strong wastewaters are mixed with other weaker wastewaters for treatment. As a consequence it is becoming a necessity that the strong wastewaters are treated by themselves. In industrial applications, treatment of strong wastewaters is at present mostly based on incineration and oxidation systems. Incineration of waste with very high concentration of organic matter, whether it be dissolved or not, is a long established industrial process. More and more, incineration is being considered as a treatment option for wastewaters with low to medium concentrations of dissolved organic matter and also strong wastewaters containing other difficult to destroy solutes.
The major advantages of such prior art treatment methods include the following:
1) The undesirable solutes are totally destroyed
2) The process is non-selective in that all and every undesirable solute present in the wastewater are treated, that is destroyed
The major disadvantages of such prior art waste treatment methods include the following:
1) The process consumes a large amount of energy when treating waste waters with low to medium concentrations of organic material.
2) The capital cost of the incineration apparatus is high particularly due to the need to minimize emissions to the atmosphere which if not dealt with adequately will cause further environmental issues.
Alternatives to incineration for the destruction of the undesirable solutes in strong waste waters are so called Advanced Oxidation Processes. There are several advanced oxidation processes that are known to us that are used to treat strong waste waters which all rely on the same basic principle. Advanced oxidation processes are designed to ensure that a population of hydroxyl radicals is generated.
Once the hydroxyl radicals, which are a very strong oxidant, are generated they attack virtually all organic species dissolved in the wastewater. Processes that are included in the Advanced Oxidation Processes are wet oxidation, wet air oxidation, wet peroxide oxidation, Fenton's reaction, ozonation and electrochemical oxidation, plus several others which to a large extent are variants of the foregoing. The main difference between these processes is in the manner in which they form hydroxyl radicals. The prior art Advanced Oxidation Processes known to us have certain advantages and disadvantages.
Wet oxidation and wet air oxidation processes have been used for many years to treat strong waste waters. The major advantages of such prior art oxidation methods include the following:
1) Wet oxidation and wet air oxidation will reliably destroy a proportion of all of the undesirable solutes in strong waste water, both organic and inorganic solutes, regardless of the chemistry of the waste water
2) The process consumes less energy than incineration when treating waste waters with low to medium concentration of contaminants
The major disadvantages of such prior art oxidation methods include the following:
1) Wet oxidation and wet air oxidation will not achieve 100% destruction of the undesirable solutes present in the strong wastewater and the treated waste water will normally require post treatment, for example in a biological treatment system, before disposal2) Wet oxidation and wet air oxidation systems are operated at high temperature and pressure and as a consequence the capital investment cost is high. The use of a catalyst to increase the rate of reaction has been introduced as an enhancement of the process which has resulted in the lowering of operating temperatures with a subsequent decrease in the capital cost, however operating costs have increased.
Wet peroxide oxidation systems have been utilized to treat certain strong waste waters. In this process hydrogen peroxide is encouraged to decompose to generate hydroxyl radicals which then go on to initiate the desired chain reactions necessary to destroy the undesirable solutes. The major advantages of such prior art oxidation methods include the following:
1) The process is practiced at lower temperature and pressure compared to wet oxidation and wet air oxidation so as a consequence the capital cost of the system is lower
The major disadvantages of such prior art oxidation include the following:
1) Wet peroxide oxidation will not normally achieve 100% destruction of the undesirable solutes
2) The process uses a significant amount of hydrogen peroxide which means that operating costs are high
3) The wet peroxide oxidation process has to be carefully controlled
4) The performance of the process is dependent upon the strong wastewater chemistry
Other Advanced Oxidation Processes known to us that have been practiced in industry to treat strong wastewaters are Fenton's Reaction, modified-Fenton's Reaction, and ozonation either by itself at high pH or in combination with other processes or reagents such as ultraviolet light or hydrogen peroxide.
The aforementioned processes are very different from each other but they all are practiced at low temperature and pressure and as a consequence have lower capital investment costs than the hot Advanced Oxidation Processes
The advantages of such prior art oxidation processes include:
1) The aforementioned processes are very different from each other but they all are practiced at low temperature and pressure and as a consequence have lower capital investment costs than the hot Advanced Oxidation Processes
The disadvantages of such prior art oxidation processes include the following:
1) All three processes and their variants reliably achieve only partial destruction of the undesirable solutes present in the strong waste waters
2) The performance of each of the processes is very dependent upon the chemistry of the strong waste waters. There are several oxidation termination reactions that can occur in each of the processes which are highly influenced by the chemical analysis of the strong waste waters being treated3) In the case of the Fenton's Reagent and modified-Fenton's Reagent chemical usage is very high. In the case of ozonation energy consumption is high. As a consequence these processes have high operating costs and in reality the processes are only really suitable for treating wastewaters with low concentrations of the undesirable solutes.
So there are certain common disadvantages with the aforementioned prior art oxidation processes which can be summarized as:
1) They only achieve partial destruction of the undesirable solutes present in the strong waste waters.
2) The overall cost of ownership when computed considering both the capital cost and the operating cost of the systems is high.
Another Advanced Oxidation Process known to us is electrochemical oxidation. This process has characteristics which point to some particular advantages when compared to other prior art oxidation processes if specific electrode materials are utilized. In particular boron doped diamond has been shown to exhibit very beneficial and helpful properties such that apparatus including electrochemical cells utilizing this electrode material have demonstrated the following characteristics:
1) When operated at specific conditions, in particular when the power supplied to the cell is controlled adequately, it is possible to generate an abundant population of hydroxyl radicals. The generation of hydroxyl radicals is controlled by conditions at the surface of the electrode and not by the chemistry of the waste water or any precursor chemical reaction.2) The electrochemical oxidation process has proved capable of destroying virtually any dissolved organic matter3) It is possible to destroy inorganic species such as sulfides, ammoniacal nitrogen4) The electrochemical oxidation process is not particularly affected by the chemistry of the wastewater5) The undesirable solutes can be, to all intents and purposes, totally destroyed.
Nonetheless the electrochemical oxidation processes known to us for treatment of waste waters exhibit major shortcomings and deficiencies. Such shortcomings and deficiencies include:
1) Most if not all materials used for the electrodes in the electrochemical cells are susceptible to fouling by certain organic and inorganic species that are present during the oxidation of wastewaters. The fouling that occurs has been shown to inhibit the oxidation process which results in a decrease in the rate of destruction of the undesirable solutes, and often causes the complete termination of the process. The presence of the fouling has the consequence of increasing the electrode area required to treat the waste water, which increases the capital cost of the system often making the process economically nonviable. If the fouling terminates the process then the result is incomplete destruction of the undesirable solutes.2) The electrical energy consumed by electrochemical oxidation when destroying the undesirable solutes present in the wastewaters has proved to be high, such that the economic viability of the process is questionable.
Thus a continuing demand exists for a simple, effective, and inexpensive process which can reliably treat strong wastewaters to substantially achieve total destruction of the undesirable solutes present in such wastewaters. It would be desirable to achieve such treatment in equipment that requires a minimum of maintenance. In particular, it would be desirable to lower both operating costs and capital costs for strong wastewater treatment systems as is required in various industries, such as oil processing, chemical production, pharmaceuticals, and municipal waste disposal.
Clearly, if a new waste water treatment process were developed and made available that utilizes the potential advantages of electrochemical oxidation whilst addressing the major disadvantages of prior art processes and minimizing them it would be of significant benefit. In summary, an economically important new strong waste water treatment system would necessarily offer some (if not most) of the benefits of electrochemical oxidation using the most effective electrode materials available whilst at the same time any such new process must be capable of effectively coping with the problems which beset all advanced oxidation processes and in particular those methods based on electrochemical oxidation.